Huntington's Disease is an inherited (genetic) disease characterized by changes in movement and intellect that usually begin in adulthood. This observational study will examine periodically participants without definite signs of HD who are considered "at risk" for developing the illness because they have (or had) a parent or sibling with HD. Little research has been done in individuals who are at risk for developing an inherited disease (like the son or daughter of a person with HD). Before we can test experimental drugs in healthy individuals who carry the HD gene, we have to address some important issues involving all persons at risk for HD, whether or not they have inherited the abnormal gene. Some questions we hope to answer on PHAROS include: - In a group of people who are at risk for HD, how many will develop signs of the illness over a three year period? - What are the earliest signs of the illness, and when do they start? - How accurate are the measurements that researchers use in detecting the onset of HD? - What factors influence the age at which a person carrying the HD gene develops the illness?